Currently, wireless communication systems such as mobile telephone systems and wireless LANs (Local Area Networks) are widely used. In one-to-many mobile communication systems wherein a wireless base station is able to communicate with a plurality of mobile stations, a mobile station, for example, first accesses a wireless base station to establish a connection. Then, using the connection thus established, the mobile station and the wireless base station exchange data with each other. The wireless base station has a variety of communication control functions such as the allocation of bandwidths to individual connections.
In order to save electrical power, some mobile stations have a sleep mode function (see The Institute of Electrical and Electronics Engineers (IEEE), “IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems”, IEEE 802.16e-2005, 2006-02-28, for example). In the sleep mode, a sleep interval and a listening interval (non-sleep interval) are set with respect to the connection. The mobile station stops radio signal processing in the sleep interval, and restarts the radio signal processing in the listening interval. The listening interval can be set so as to occur intermittently. When entering the sleep mode, the mobile station requests a desired sleep interval to the wireless base station. Where the request of the mobile station is accepted, the wireless base station executes control so that data may not be exchanged with the mobile station during the sleep interval.
Meanwhile, in some mobile communication systems, a wireless base station and a mobile station use a plurality of frequency bands in parallel to communicate wirelessly with each other. Such a wireless communication method is often called multicarrier operation or multiband operation (see The Institute of Electrical and Electronics Engineers (IEEE), “IEEE 802.16m System Description Document [Draft]”, IEEE 802.16m-08/003r7, 2009-02-07, for example). The mobile station is equipped with a plurality of wireless units for processing radio signals and thus can communicate wirelessly in parallel by using a plurality of frequency bands. The multiple frequency bands used in this case may be discontinuous.
In connection with the use of multiple frequency bands, a technique has been proposed in which, when allocating a frequency band to a mobile station, a wireless base station allocates a high-frequency band in preference over the other frequency bands (see International Publication Pamphlet No. WO 2006/088082, for example). There has also been proposed a technique in which a transmitter determines parameters to be used for the transmission process, in accordance with each user's requested QoS (Quality of Service) and the variation characteristics of individual frequency bands (see Japanese Laid-open Patent Publication No. 2006-94005, for example).
Let us consider the case where the wireless base station and the mobile station communicate wirelessly with each other by using a plurality of frequency bands. One of methods allowing the mobile station to save electric power is the aforementioned method in which the mobile station requests the wireless base station to set the sleep interval. Another method is also conceivable in which the mobile station requests the wireless base station to make one or more of the frequency bands inactive (unused). Where these two independent methods are used in combination to save electric power, however, trials and errors are liable to occur, giving rise to the problem that the overhead of communication control increases.
Specifically, the designation of a sleep interval by the mobile station can possibly be rejected by the wireless base station, and such a situation occurs when the bandwidth for the listening interval fails to be secured, for example. In such cases, if the frequency band or bands to be activated (to be used) are changed, the designation of the sleep interval can possibly be permitted. Thus, the mobile station has to designate the frequency bands to be activated or deactivated as well as the sleep intervals of individual connections one after another in order to select a desirable power saving method from among the settable combinations, entailing trials and errors.